1. Field of the Invention
The invention relates to a metal halide lamp, especially a metal halide lamp of the short arc type which is used as a light source of a television set of the liquid crystal projection type.
2. Background of the Invention
For a fight source of a television set of the liquid crystal projection type, a metal halide lamp with high efficiency and good color rendering has been recently used. For a lamp of this type, a lamp is often used in which halides of rare earth metals, such as dysprosium, neodymium and the like, as well as a halide of cesium, are encapsulated.
Encapsulation amounts of these materials are often greater than or equal to 0.4 micromole/cm.sup.3 fluorescent tube volume for the rare earth metal halides and greater than or equal to 0.2 micromole/cm.sup.3 fluorescent tube volume for the cesium halide.
Due to the requirement of high brightness, this lamp is operated with a high lead of 35 W/cm.sup.2 to 80 W/cm.sup.2. The temperature of one fluorescent tube wall is therefore greater than or equal to 900.degree. C. In luminous operation of the lamp with a duration of several hundred hours, therefore, milky cloudiness occurs on the tube wall.
Since the occurrence of milky cloudiness greatly degrades the light efficiency, it can be stated that, essentially, the occurrence of the milky cloudiness marks the end of the service fife of the lamp.
As a process for preventing the occurrence of milky cloudiness, conventionally, a cesium halide was added; however, the action was not satisfactory enough. For example, in luminous operation of a conventional lamp for 2000 hours, the screen fight flux drops to less than or equal to 50% of the screen light flux at the start of luminous operation; this undoubtedly occurs as the result of the influence of the milky cloudiness.
On the other hand, as a process for operating the above described metal halide lamp, ordinarily alternating current luminous operation using the line frequency (50 Hz-60 Hz) and luminous operation using acutely angular waves with roughly 50 Hz to 500 Hz are used in practice. Furthermore, luminous operation using a direct current is proposed.
In luminous operation using direct current, it is necessary to induce convection in a suitable amount within the lamp in order that polarization of the emission material present inside the lamp is prevented in a certain area, which can also be designated concentration or accumulation on a certain side. Convection is generally caused by heat originating from an anode, the lamp being arranged such that the arc axis is perpendicular to an upper electrode and a lower electrode.
On the other hand, by analyzing the milky deposit adhering to the fluorescent tube, it was found that it had formed by accumulation of microcrystalline silica (crystals which are called crystobalite) with a diameter of roughly 1 micron. The reason for the formation of this microcrystalline silica is presumably the following:
The rare earth metals which are encapsulated in the fluorescent tube are usually in a state in which they are bound to a halogen in the vicinity of the fluorescent tube. However, these rare earth halides vaporize when the temperature of the tube wall rises to roughly 850.degree. C. If these vaporized rare earth halides occur in an arc with a high temperature, they are converted by dissociation into rare earth atoms, and by ionization or excitation of these rare earth atoms emission is effected. When the rare earth atoms within the arc, as a result of convection or diffusion up to one part with a low temperature, reach the vicinity of the tube wall, they are converted by recombination with the halogen back into rare earth halides.
Only a small part of the rare earth ions or rare earth atoms however are not recombined with the halogen, but can adhere to the fluorescent tube wall in one state of the ions or atoms.
It is assumed that these rare earth ions influence the silica of the quartz glass with a very high probability, and that the rare earth atoms do so with a certain probability and thus convert the silica into silica crystals in a microcrystalline state.
The possibility that the rare earth ions or the rare earth atoms reach as far as the fluorescent tube wall presumably increases, the smaller the distance between the arc and the tube wall and the higher the temperature of the tube wall. This means that milky cloudiness occurs more frequently, the higher the tube wall load of the lamp; this corresponds to an empirical fact.